Thanks chelle - sadly it didn't respond to the paper clip treatment. I can only think that the whole drawer needs to be taken out and probably cleaned. Do you have another suggestion?
When you used the paper clip you should have initially felt some resistance to the push action and then felt it slide forward by using a bit more pressure, or even heard a slight click as it tried to release the lock mechanism.
If not, try again. I sometimes use a needle althogh a paper click should be work.
If the drive is sticking you may also need to gently lever the front of the drive with a small screw driver at the same time.
Thanks chelle & pb - Hooray! Really persevered this time and tried a very long thin steel crochet hook which went deeper into the small hole beneath the drawer and hey presto - success!!! But, it has taken me all the morning and into the afternoon (with lunch inbetween) to succeed (hate being beaten). Once again, many thanks, trish.